Charging 24v Battery Bank From 12v Source

[robertglines1]

been watching this: I have very little 24V requirements. wiper & defroster fans.--The truck engine came with a 12V alternator & starter. The ddec is 12 volt.  I have 2 series 31's in parallel=12 volt for starter. I then need 24V so I added another 31 battery and achieved 24 volt  The starter and 12 volt tap is before the #3 battery which then supplies a 24 volt main.  question is how to charge batt #3.  The alternator supplies 13.87 V to batteries 1 & 2.  Observation: the charge in # 3 is increasing  but does not show  13.87 volt--It reads 12.7  up from 12.5  in 20 minutes idle time.  The dash gauge shown 27 volt with engine running.  all things hooked to either 12v or 24 volt have the correct readings. I forgot there is a 12V master switch also.  Comments please--do I need to add a charge system to #3?   Bob

[bevans6]

It sounds like there is nothing to charge battery #3, if there is you didn't mention it.  You need something to charge it, either of the systems that have been discussed here.  Since it is essentially half the size of the other pair of batteries whatever you do will probably over-charge it.  If you really have very small 24v requirements I would use a voltage converter to create 24v from 12v and have a stand-alone 24v bank - basically add a fourth battery.  They need to be the same as each other but they don't need to be group 31's, unless that is easier for you.

Are all your other bus systems converted to 12v, like lights and so on?

Brian

[robertglines1]

everything except wiper motor and defroster fans is 12 volt. That what I thinking about # 3  charging.  I have a vanner 100 amp but have not thought that thru yet(not in system).  I have a 4 bank battery charger and it worked out .(On another project-)- 4ea 10amp 12 volt leads(like in bass boat) Guess the light just came on! Trolling motor was 24volt in it.. has 4 each 12 volt batteries. I took wiper motor to re-builder and they wanted $400 to change/duplicate it to 12 Volt plus I would have to replace defroster motors also.. I have no house bank and don't plan on it. Thanks for helping me think this thru.. Charge rate on # 3? would it be effected by charging 1&2 which are hooked up to 12V alt on engine( If separate charge source)..Bob

[Audiomaker]

Shelled, what about the fact that in a 24 volt system the 0 - 12v battery has it's positive post connected to the negative post of the 12 - 24v battery?  You've just connected your negative to your positive and created a dead short in your charging system.  I could say trust me, I've been doing this for 40 years, but I won't.  You are missing the point that while the system is charging it is still an operational 24 volt system.  Your approach is to just rewire the batteries as a 12 volt parallel bank, and does not satisfy the requirement.

Brian

I must concur, however you could charge from multiple 12v sources in a series array if they were totally isolated.... which is an interesting prospect.

For instance, 4 12v battery chargers (or alternators) could charge a 48v array.

Simply wiring parallel to each battery in a series string would not work, and as described...would make a heck of a short.

Also, having 4 100% isolated 12v alternators (2 for a 24v array), might look cool, but it's impractical.

But what about isolators?   I wonder if there is any way to wire a dual battery isolator to do this (actually a pair because you'd need it for the ground too).

Sounds like some sparky fun to find out!

[bevans6]

I'll be honest - charging batteries isn't rocket science, all you need to do is figure out the charge rate, how to monitor the charge level ( easy with a modern charger or alternator, they automatically monitor charge level) and then balance the system.  I bet the boat guys have this covered - there is probably an off-the-shelf product that does exactly what Audiomaker suggests - multiple independent charging systems from a 12 or 24 volt input.  I could easily design and put one together from off the shelf products, but if it isn't productized you are paying retail for all the parts so you pay a lot.  The real key would be to have 12v input, 12 volt output (basically passing through the alternator output voltage) but with all the outputs having floating grounds so they can be connected to individual batteries without worrying about shorts, and each having independant smart charger capability built in to control the charge rate for each battery.

Here is an easy way to go.  Get a cheap 1000 watt pure sine inverter, two 12 volt 20 amp smart chargers, run the inverter from the APU (or bus) alternator and connect one charger to the 0 - 12v set and one charger to the 12 - 24 volt set.  Total cost under $250 bucks.  Robert, you could do that and cut it in half, all you need to do is charge the 12 - 24 volt single group 31 you have.  A little inverter that is powered by the 12v start batteries (and the bus alternator when running), a small smart charger and a switch to control it so you aren't charging the battery when the engine is off.   A 20 amp smart charger draws around 300 watts from an inverter, you do want a pure sine inverter to run the charger though.  It's counter-intuitive to use an inverter to power a battery charger but in the context of powering from an APU or bus alternator, it makes a lot of sense.


Brian

[Lostranger]

Glad I asked. Thanks for all suggestions. I especially love background explanations.

Still pondering this one. We have some need for 12v in bus (CB, cable modem, cell phone chargers), but we use a step down. I was leaning toward changing to a 24v alternator and charging APU starter battery with a step down converter and isolator. The complication with this approach is that the APU brain and systems work on 12v. This includes engine fan, condenser fan and evaporator fans. Our 12v need will be larger than before. Guess I'll have to keep the 12v alternator.

I'm still in the process of harvesting the APU and its systems from a wrecked tractor. Soon as I can get home, I'll look into the possibility of adding a second (24v) alternator. I'd like the idea of having an alternator dedicated to charging the main bank.

Someone on the thread asked if the APU will be the only means of charging the main battery bank. Our primary charging source is solar.

Jim

[Lostranger]

Brian, your post came up while I was typing. I'll read it slowly later today when I can think about it. Thanks for chiming in.

Jim

[Audiomaker]

I'll be honest - charging batteries isn't rocket science, all you need to do is figure out the charge rate, how to monitor the charge level ( easy with a modern charger or alternator, they automatically monitor charge level) and then balance the system.  I bet the boat guys have this covered - there is probably an off-the-shelf product that does exactly what Audiomaker suggests - multiple independent charging systems from a 12 or 24 volt input.  I could easily design and put one together from off the shelf products, but if it isn't productized you are paying retail for all the parts so you pay a lot.  The real key would be to have 12v input, 12 volt output (basically passing through the alternator output voltage) but with all the outputs having floating grounds so they can be connected to individual batteries without worrying about shorts, and each having independant smart charger capability built in to control the charge rate for each battery.

Here is an easy way to go.  Get a cheap 1000 watt pure sine inverter, two 12 volt 20 amp smart chargers, run the inverter from the APU (or bus) alternator and connect one charger to the 0 - 12v set and one charger to the 12 - 24 volt set.  Total cost under $250 bucks.  Robert, you could do that and cut it in half, all you need to do is charge the 12 - 24 volt single group 31 you have.  A little inverter that is powered by the 12v start batteries (and the bus alternator when running), a small smart charger and a switch to control it so you aren't charging the battery when the engine is off.   A 20 amp smart charger draws around 300 watts from an inverter, you do want a pure sine inverter to run the charger though.  It's counter-intuitive to use an inverter to power a battery charger but in the context of powering from an APU or bus alternator, it makes a lot of sense.


Brian

That's pretty similar to what I posted on the 1st page of this thread... just go from the 12v APU battery to an inverter in the 1500w range (sine), and then to a charger (just use a single 24v smart charger).
The Xantrex ProWatt SW is sine wave, and can be turned on and off by 12v+, so you use the run signal of the APU to switch it on.
The inverter runs only when the APU is running...the charging is "smart", and you get an extra inverter circuit to anything you might like to power only while the APU is running. (and a 12vdc battery source as well instead of tapping the 24v array).

The downside is you lose a little efficiency in the conversions so your cost per amp hour goes up maybe 20%, but if it's a secondary system to solar and also used for heat and A/C mechanically, then the efficiency increases on that side more than make up for it.

Sean

[shelled]

Brian,

Thanks for sharing, you have added a lot to my understanding.

You mentioned a theoretical system that would wrap all this up in one unit.  My nephew works for a rural wireless ISP and they use a controller for their solar power systems that sounds like it provides individual charging control for multiple non-identical batteries and multiple voltage outputs from multiple DC and AC inputs.  I'm hoping to get a look at this when I get a chance to visit him later this summer.  Knowing how cost conscious these folks are, I'm also hoping it's affordably priced.

edward

[Oonrahnjay]

     And, as a total aside from this but I hope it's helpful, a friend of mine has a bus charter company and if he has a hard-starter, he takes an ordinary 12V  "jump box" (the plug in type that looks kind of like a briefcase with jumper cables coming out of it) and attaches it to one of the 12V batteries in a 24V pair.  It's amazing how a just-barely-turning-over engine without will suddenly jump into life when that single box is added.   I might not work *all* the time but it is amazing when it does, which seems to be usually.

[bigred]

Hey Guys:Don't know if this will be any help or not,but my Prevost has a 12/24v system.It has six house battery's and 24v and a 12v that they refer to as "isolators"?.This whole system is charged off a, as far as I can tell, a 12v alternator.A very large one!Some of the guy's on the Prevost web site can probably shed more light on this than I can.On the Prevost, this is a can of worms as you never know which voltage you are dealing with until you check it with a volt meter!

[robertglines1]

Bigred: red wires are 24 volt and yellow 12volt on prevost wiring. # tatoo on wire will give you what they are for.  prevostcar.com  under wiring diagram  gives you the code.  Bob